In general, a particle beam therapy system includes: a beam generation apparatus for generating a charged particle beam; an accelerator connected to the beam generation apparatus, for accelerating the generated charged particle beam; a charged-particle beam transport system for transporting the charged particle beam emitted from the accelerator after being accelerated therein up to a setup given energy; and a particle beam irradiation apparatus placed on the downstream side of the beam transport system, for irradiating the charged particle beam to an irradiation target.
When the beam is extracted using a resonant extraction scheme from a synchrotron as the accelerator, or when the beam is extracted from a cyclotron as the accelerator and a collimator is being provided in the beam transport system, the particle distribution in cross-sectional direction of the beam results in a profile in which the number of charged particles decreases drastically at the ends, like a rectangular shape, for example. In the case where an irradiation field is formed by scanning the beam like in the spot-scanning irradiation method or the raster-scanning irradiation method, when the number of charged particles decreases drastically at the ends of the dose distribution as shown in FIG. 2 and FIG. 3, a following problem arises. FIG. 2 and FIG. 3 are diagrams each illustrating positional displacement and robustness of a beam. FIG. 2 corresponds to a case where the ends of the particle-beam distribution are moderate, and FIG. 3 corresponds to a case where the ends of the particle-beam distribution vary steeply. In FIG. 2 and FIG. 3, the abscissa represents a beam scanning direction X in the irradiation target, and the ordinate represents a dose (charged-particle number). Dose distributions 81, 86 each indicated by a broken line, are each a dose distribution at one irradiation position in the spot-scanning irradiation method. Dose distributions 87, 88 in FIG. 3 correspond to a case where the beam profile and the beam irradiation position are as planned and thus there is no displacement in the beam-irradiation position. Dose distributions 89, 90 in FIG. 3 correspond to a case where the beam profile and the beam irradiation position are not as planned and a displacement occurs in the beam-irradiation position. The respective dose distributions 87, 89 in FIG. 3 are each a dose distribution at each of the irradiation positions, and the dose distributions 88, 90 in FIG. 3 are each an integrated dose distribution in the overall irradiation field. As shown in FIG. 3, in the case where the beam having a dose distribution whose ends vary steeply is irradiated, the flatness of the dose distribution 90 in the formed irradiation, field is largely deteriorated in response to a displacement in the beam profile or in the beam-irradiation position.
In contrast, as shown in FIG. 2, in the case of the distribution, like a Gaussian distribution, in which the charged-particle number variation at the ends is moderate, it is possible to make the flatness of the dose distribution 85 in the irradiation field better than, that in FIG. 3. Like in FIG. 3, dose distributions 82, 83 in FIG. 2 correspond to a case where the beam profile and the beam irradiation position are as planned and thus there is no displacement in the beam-irradiation position. Like in FIG. 3, dose distributions 84, 85 in FIG. 2 correspond to a case where the beam profile and the beam irradiation position are not as planned and a displacement occurs in the beam-irradiation position. The respective dose distributions 82, 84 in FIG. 2 are each a dose distribution at each of the irradiation positions, and the dose distributions 83, 85 in FIG. 2 are each an integrated dose distribution in the overall, irradiation field. As shown in FIG. 2, in the case where the beam having the distribution, like a Gaussian distribution, in which the charged-particle number variation at the ends is moderate, is irradiated, the flatness of the dose distribution 85 in the formed irradiation field is improved against the displacement in the beam profile or in the beam-irradiation position, in comparison with the dose distribution 90 in FIG. 3.
In the case of forming an irradiation field by scanning the beam, when the charged-particle number decreases drastically at the ends of the dose distribution, the robustness against a change in the irradiation position or beam profile, or a displacement in the irradiation position is impaired, so that it becomes difficult to form the irradiation field that is flat in the dose distribution. For example, when the charged-particle number decreases drastically at the ends of the dose distribution, it is necessary to control the irradiation position and the size of the beam, for example, up to 0.1 mm or less.
In Patent Document 1, a charged particle irradiation system, is described that modifies the charged particle distribution, which corresponds to a case where the charged-particle number decreases drastically at the ends of the dose distribution and in which the emittance ellipses in an X-direction and a Y-direction are asymmetric to each other, into a Gaussian distribution both in the X-direction and the Y-direction. The charged particle irradiation, system of Patent Document 1 includes, in its beam transport system extending from the accelerator to an irradiation section, an up-stream-side electromagnet comprising four quadrupole electromagnets, a scatterer provided downstream of the electromagnet, and a downstream-side electromagnet provided downstream of the scatterer and comprising four quadrupole electromagnets. In the charged particle irradiation system of Patent Document 1, the beam whose emittance ellipses in the X, Y-directions are asymmetric to each other is modified by the upstream-side electromagnet so that position components X and Y in the emittance ellipses are the same, and then the angle components in the Y-direction are expanded by multiple, scattering using the scatterer to thereby make same the emittances in the X, Y directions. Thereafter, the beam is adjusted to have an intended beam diameter by adjusting the emittances in the X, Y-directions using the downstream-side electromagnet.